This invention relates generally to rotation sensors and particularly to fiber optic rotation sensors. Still more particularly, this invention relates to apparatus and methods for modulating light signals in a fiber optic rotation sensor to maximize operational efficiency and minimize errors.
A fiber optic ring interferometer typically comprises a loop of fiber optic material having counter-propagating light waves therein. According to the Sagnac effect, the wave traveling in the direction of rotation of the loop has a longer transit time through the loop than the wave traveling opposite to the direction of rotation. This difference in transit time is seen as a shift in the relative phases of the waves. The amount of phase shift depends on the rotation rate. After traversing the loop, the counter-propagating waves are combined so that they interfere to form an optical output signal. The intensity of the optical output signal varies as a function of the type and amount of interference, which is dependent upon the relative phase of the counterpropagating waves. The optical output signal produced by the interference of the counter-approaching waves varies in intensity as a function of the rotation rate of the loop. Rotation sensing is accomplished by detecting the optical output signal and processing it to determine the rotation rate as a function of the phase shift.
A fundamental property of fiber optical ring rotation sensors is reciprocity. Ordinarily, any disturbance of the optical path affects both waves in the same way, even if the two waves are not subjected to the disturbance at exactly the same time or in the same direction. There are however, some disturbances which vary in time with a period comparable to the propagation time of the waves through the sensing loop. There are also nonreciprocal disturbances which do not have the same effect on the waves. The nonreciprocal disturbances are the Faraday Effect and the Sagnac Effect. These two types of disturbances which do not obey the reciprocity principle.
If no nonreciprocal disturbances appear in the counterpropagating waves, the phase difference between the two waves when they are recombined in the separating and mixing coupler is zero. The detection and processing apparatus respond to signals indicative of the optical power of the composite wave obtained after the counterpropagating waves have been mixed together. This power may be broken down into a constant component and a component proportional to the cosine of the phase shift. The component proportional to the cosine of the phase shift appears in the signal only when there are nonreciprocal disturbances in the optical path of the waves.
In the measurement of low amplitude disturbances, which correspond to low rotation rates, the component containing the cosine of the phase shift term is very small since the phase shift, .DELTA..phi., is nearly zero. It is then necessary to add a fixed additional phase shift or nonreciprocal bias to the waves to increase the sensitivity of the measurement of the phase shift. One situation of interest is where the new phase shift measured is .DELTA..phi.'=.DELTA..phi.+.pi./2. In this case, the sensitivity is maximized since the term to be measured is proportional to cosine (.DELTA..phi.+.pi./2), which is proportional to sine .DELTA..phi.. For small .DELTA..phi., the sine term has maximum slope, so that small changes in .DELTA..phi. are measurable.
There have however been difficulties in implementing a device that introduces a sufficiently stable nonreciprocal bias to be usable in a navigation grade rotation sensor.
Arditty et al. in Canadian Pat. No. 1,154,955 disclosed a process for shifting the operating point of a ring interferometer without requiring either a nonreciprocal bias or a great stability of the phenomena used to shift the operating point. That patent discloses a process for modulating the phase of waves in a ring interferometer that comprises the steps of forming a ring wave guide in which two electromagnetic waves travel in opposite directions, providing a source of electromagnetic energy, separating and mixing the electromagnetic waves in the ring and detecting the interference of the waves in the ring to determine the phase difference between them. Arditty et al. further disclose periodic and symmetric modulation of the phase of the waves with a period function .phi.(t)=.DELTA..phi.(t+2.tau.) where .tau. is the time for each of the waves to travel over the path defined by the ring. Arditty further discloses detecting the phase difference at the frequency 1/2.tau. and discloses a phase modulation device for implementing the process.